Document Type : Original Research Paper
Authors
^{1} Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
^{2} Nanotechnology Research Institute, Faculty of Chemical Engineering, Babol Noshirvani University of Technology, Babol, Iran
^{3} Department of Chemical Engineering, Mazandaran University of Science and Technology, Mazandaran, Iran
Abstract
Keywords
INTRODUCTION
In recent years, there has been an increasing interest in adsorption of various molecules [1-6] and developing gas storage systems for various applications such as CO_{2} capture or H_{2} storage [7-10]. By the same token, the metal-organic framework (MOF) family of materials have attracted particular attention due to their fascinating sorption properties [11-13]. In these materials, inorganic clusters (metals) and organic ligands can be connected to adopt a wide range of different structures, which open the possibility of tuning host-guest interactions in order to explore molecular sieving and adsorption properties unavailable in other materials.
One of the most important MOFs is Zn_{4}O_{13}C_{24}H_{12} framework called as MOF-5, which was invented in 1999 [14]. This framework has potential applications for H_{2} storage, CO_{2} capture, and catalysts [15]. The MOF-5 consists of Zn_{4}O nodes which are linked to terephthalate anions (1,4-BDC = 1,4-benzenedicarboxylate) groups to form a porous material (Fig. 1(a)) [14]. It has been recognized that the MOF-5 occupies either cubic (C-MOF-5) or tetragonal (T-MOF-5) structure [16-20]. Cubic MOF-5 (with ultra-high BET surface area up to 3400 m^{2}/g) has widely been synthesized by using a“diffusion” method [14] and tetragonal MOF-5 (with a medium surface area) can be synthesized via“direct mixing” approach [16].
The tetragonal MOF-5 is a distorted cubic MOF-5 [18]. Furthermore, this structure distortion was attributed to the presence of ZnO species extra phase and solvent in the pores [15,18-20]. Zhang and Hang Hu [15], showed that the composition (Zn_{4.28}O_{12.8}C_{24}H_{11.3}) of cubic MOF-5 sample is consistent with the stoichiometrical formula of Zn_{4}O_{13}C_{24}H_{12}. In contrast, the composition of the tetragonal MOF-5 sample with the calculated formula of Zn_{4}O_{13}C_{24}H_{12.6}(ZnO)_{1.59}(H_{2}O)_{1.74} was very different from the stoichiometrical formula of Zn_{4}O_{13}C_{24}H_{12}, because ZnO and H_{2}O were present. The presence of H_{2}O might be due to the adsorption of H_{2}O when it was exposed to air. The formation of ZnO species is due to the reaction between zinc nitrate and H_{2}O_{2} during the synthesis of tetragonal MOF-5.
In our previous work, we have shown that the T-MOF-5 had a lower surface area, lower porosity, smaller and more uniform pore size, and more ZnO molecule than C-MOF-5 [21]. Also in another work, we have synthesized and characterized the C-MOF-5 and T-MOF-5 as adsorbents for H_{2} and CO_{2} adsorption [22]. We found that the CO_{2} adsorption capacity of C-MOF-5 is greater than that of T-MOF-5 and the H_{2} adsorption capacity of C-MOF-5 is less than that of T-MOF-5 (at 298 K and 25 bar). This behavior was attributed to more ZnO units in T-MOF-5 than C-MOF-5.
According to literature, adsorption of small molecules in MOF-5 occurs first in a site near the Zn cluster in the large pores (site A’ in Fig. 1(b)) [23-27]. Sarmiento-Perez et al [28] introduced a similar site that is present in the smaller pore (site B’ in Fig. 1(b)). They showed that the site B’ in Fig. 1(b) is the main site for CO_{2}.
As reported in the literature, calculation of the chemical properties of MOF-5 using a representative fragment (such as an organic ligand) is in good agreement with the experimental results [29-31]. Considering the large size of cells in the MOF-5 structure for a quantum mechanics calculation, the model system should be decreased in size. The size reduction can be achieved by separating the H_{2}BDC linker. Accordingly, in this paper, we considered the adsorption behavior of CO_{2} and H_{2} molecules on the surface of C-MOF-5 (H_{2}BDC as a representative section) and T-MOF-5 (ZnO-H_{2}BDC as a representative section).
In the current study, we examined the adsorption of H_{2} and CO_{2} on the surface of H_{2}BDC and ZnO-H_{2}BDC (as a representative section of C-MOF-5 and T-MOF-5 nanostructures) using DFT calculations. First, we explored the geometric and electronic properties of H_{2}BDC surfaces upon decoration of ZnO (ZnO-H_{2}BDC) by examining binding energies, the charge transfer, density of states, dipole moments and adsorption geometries for the probable position. Then, the relaxed H_{2}BDC and ZnO-H_{2}BDC ligands are theoretically investigated for H_{2} and CO_{2} adsorption. In other words, we investigate in detail, the effect of more ZnO molecule in T-MOF-5 nanostructure than C-MOF-5 nanostructure on H_{2} and CO_{2} adsorption. We have achieved first-principles calculations to walk around the interaction behavior of the H_{2} and CO_{2} with H_{2}BDC and ZnO-H_{2}BDC.
Computational strategy
Geometry optimization, the density of states (DOS), and energy analyses were performed to investigate the adsorption phenomena using Gaussian 09 program package [32] with density functional theory (DFT) at B3LYP/6-31G (d,p) functional/basis set. The 6-31G(d,p) basis set is good for general calculations, besides the B3LYP density functional has been known appropriate for nano-structure studies. The temperature and pressure for all calculations were 298.15 K and 1.0 atm respectively.
A number of possible orientations are considered for H_{2}BDC, ZnO-H_{2}BDC, CO_{2}-H_{2}BDC, H_{2}-H_{2}BDC, CO_{2}-ZnO-H_{2}BDC, and H_{2}-ZnO-H_{2}BDC, but all input geometries resulted in optimization of limited structures, according to Fig. 2.
The adsorption Energy of ZnO on the H_{2}BDC is calculated by:
where is the energy of H_{2}BDC interacting with the ZnO and is the energy of a pristine H_{2}BDC, and is the energy of pristine ZnO. The interaction energy of CO_{2} or H_{2} (indexed by CO_{2}/H_{2}) with pristine H_{2}BDC and H_{2}BDC-ZnO are calculated by:
where and correspond to interaction energy of CO_{2}/H_{2} with H_{2}BDC and ZnO-H_{2}BDC, and are energies of H_{2}BDC and ZnO-H_{2}BDC interacting with the CO_{2}/H_{2}, and is the energy of an isolated CO_{2}/H_{2}. All the mentioned energies of the equations are related to equivalently relaxed minimum energy structures. Also chemical potential () [33], hardness () [33], softness (S) [33] and electrophilicity () [33] are calculated by:
where E_{HOMO} and E_{LUMO} are the energies of HOMO and LUMO, respectively.
The charge transferring between CO_{2}/H_{2} and the surface of both H_{2}BDC and ZnO-H_{2}BDC is calculated from the varying of the charge concentration on CO_{2}/H_{2} after adsorption and an isolated spice. This calculation has been done by Mullikan charge analysis [34].
RESULTS AND DISCUSSION
Geometries of isolated pristine H_{2}BDC, ZnO-H_{2}BDC and their interaction with CO_{2} and H_{2} have optimized at B3LYP functional at 6-31G(d,p) basis set to allow them to be relaxed. Although the site C in Fig. 2 is the most attractive location for ZnO adsorption, but because this place will be occupied by ZnO molecules, as inorganic cluster, therefore in this study, to investigate the location of extra ZnO adsorption around the H_{2}BDC, the site A, B, and D in Fig. 2 were investigated. In this paper, we will discuss the strongest adsorption site and use the more stable configurations to further study (for all systems). Fig. 3 shows the optimized unit cell of H_{2}BDC and ZnO-H_{2}BDC. The values of adsorption energy, as well as some important parameters for all relaxed systems, are listed in Table 1. The geometric parameters of H_{2}BDC are in good agreement withthe already reported data [35]. According to results, the location D in Fig. 2 is the best location (in front of location A and B) for ZnO adsorption on H_{2}BDC (whit high adsorption energy, -111.5627 kJ/mol).
In order to study the adsorption of H_{2} and CO_{2} on H_{2}BDC and ZnO-H_{2}BDC, it is essential to note that there are two possible configurations for CO_{2} (O end and C end) to be optimized. In this study, we used the more stable configuration. The interaction of H_{2} and CO_{2} molecules with H_{2}BDC and ZnO-H_{2}BDC on different positions (site A, B, and D in Fig. 2) has been studied using the above-mentioned method and let them be fully optimized. Fig. 4 shows the optimized unit cell of H_{2}-H_{2}BDC and CO_{2}-H_{2}BDC in three positions (site A, B, and D in Fig. 2). In this study, we found that there are two main sites for H_{2} and CO_{2} adsorption (named site A and site B respectively) on H_{2}BDC, while the E_{ads} for adsorption of H_{2} and CO_{2} on site D, is very little (as shown in Table 1).
In fact, considering that the organic ligand in MOF-5 is an appropriate representative [29-31], therefore Fig. 4 shows the adsorption behavior of H_{2} and CO_{2} on C-MOF-5. Accordingly as noted in the previous sections, the T-MOF-5 had more ZnO molecule than C-MOF-5. The differences between two nanocrystal structures bring about the different behavior in gas adsorption. To investigate the adsorption behavior of H_{2} and CO_{2} on T-MOF-5, the adsorption study on ZnO-H_{2}BDC (according to Fig. 3 (b)) was done. Fig. 5 shows the optimized unit cell of H_{2}-ZnO-H_{2}BDC and CO_{2}-ZnO-H_{2}BDC. The H_{2} and CO_{2} adsorption energies in ZnO-H_{2}BDC are -7.6125 and -45.6750 kJ/mol, respectively. According to results, we can conclude that, at least in the organic ligand of the MOF-5, the highest and lowest adsorption of H_{2} (or CO_{2}) is attributed to the T-MOF-5 and C-MOF-5 respectively. Also by changing in structure from cubic to tetragonal, the main site for H_{2} and CO_{2} adsorption was changed. According to obtained results, the interaction of H_{2} (or CO_{2}) on H_{2}BDC and ZnO-H_{2}BDC are in the range of physisorption. That’s while the interaction between ZnO and H_{2}BDC is intermediate between chemisorption and physisorption.
Although in this study, the investigations have been done on organic ligand of MOF-5, and according to the overall structure of MOF-5 has not been analyzed, however, the obtained results for H_{2} adsorption on H_{2}BDC and ZnO-H_{2}BDC (as representative sections of C-MOF-5 and T-MOF-5) are in good agreement with experimental results obtained for H_{2} adsorption on C-MOF-5 and T-MOF-5 [22]. Also, the experimental results for CO_{2} adsorption on C-MOF-5 and T-MOF-5 [22] are in contrast to the obtained results in this study for CO_{2} adsorption on H_{2}BDC and ZnO-H_{2}BDC (little difference in the same temperature and pressure).
As noted in the previous sections and according to our previous experimental results [21] with changing the structure from cubic to tetragonal, the pore size reduced from 8.67 Å to 6.3 Å. Because the kinetic diameter of hydrogen is very small, then it can be expected that changing the pores size from cubic to tetragonal has little effect on the H_{2} diffusivity [19,20] and more effect on the bigger molecules (CO_{2} in this study). Also, Sarmiento-Perez et al [28] reported that the primary step in the abnormal CO_{2} adsorption in MOF-5 is due to synergic and significant contributions from all the three: the inorganic cluster, the organic linker and the 3-D structure of the nanopores. Therefore, the agreement between experimental results [22] and DFT theoretical results presented in this study for H_{2} adsorption is attributed to the very small kinetic diameter of H_{2}. Accordingly, the difference between experimental results (complete structure) [22] and theoretical results presented in this work for CO_{2} adsorption, is attributed to the bigger kinetic diameter of CO_{2} and high impact of 3-D structure on the adsorption of CO_{2}. However, this difference does not violate the result of CO_{2} adsorption (in this study).
We believe that the adsorption of ZnO on H_{2}BDC and also the adsorption of H_{2} and CO_{2} on H_{2}BDC and ZnO-H_{2}BDC can be interpreted by the amount of net charge transfer. The amounts of charge transferring between H_{2}BDC, ZnO, ZnO-H_{2}BDC, H_{2}, and CO_{2} were simulated at the B3LYP functional at 6-31G(d,p) basis set. The Mulliken charge analysis is essential analysis for examination the net charge transferring in molecular systems. Table 1 shows the Mulliken charge of interacted ZnO with H_{2}BDC and alsointeracted H_{2} and CO_{2} with H_{2}BDC and ZnO-H_{2}BDC. In H_{2}-H_{2}BDC-A complex, H_{2} detracts about 0.011e^{− }charges to H_{2}BDC. Reverse transfer of charge achieved in ZnO-H_{2}BDC, CO_{2}-H_{2}BDC-B, CO_{2}-H_{2}BDC-D, H_{2}-H_{2}BDC-D, CO_{2}-ZnO-H_{2}BDC, and H_{2}-ZnO-H_{2}BDC complexes whereas ZnO loses about 0.107e^{−} charge from H_{2}BDC and CO_{2} (also ZnO) loses about 0.101e^{−} (0.059e^{−}) charge from ZnO-H_{2}BDC (also CO_{2}-H_{2}BDC) and H_{2} (also ZnO) loses about 0.083e^{−} (0.058e^{−}) charge from ZnO-H_{2}BDC (also H_{2}-H_{2}BDC). Also for CO_{2}-H_{2}BDC-B and CO_{2}-H_{2}BDC-D and H_{2}-H_{2}BDC-D complexes no net charge transfer occurs.
As shown in Table 1, significant changes in electronic properties were achieved during adsorption of ZnO on H_{2}BDC. Also, according to results of Table 1, there are no significant changes in the electronic properties of the pristine H_{2}BDC surface during interaction with H_{2} and CO_{2} but significant changes in electronic properties were achieved during adsorption of H_{2} and CO_{2} on ZnO-H_{2}BDC.
Supporting confirmation was found in the measured dipole moments (µD) of all systems. The dipole moment is related to the particular property of a molecule which considers data of electronic and geometrical properties [36]. The amounts of the size and structures of the dipole moment for H_{2}BDC on ZnO-H_{2}BDC and also for H_{2} and CO_{2} on the H_{2}BDC and ZnO-H_{2}BDC are listed in Table 1. Our computations reveal that when ZnO comes close to the H_{2}BDC, the size and directions of µD will change dramatically (from 0.0012 to 2.3339). Also, we found that during H_{2} and CO_{2} interaction for all systems, the µD is raised. The µD values of the interacted pristine H_{2}BDC with H_{2} (site A), H_{2} (site D), CO_{2} (site B), and CO_{2} (site D) are equal to 0.2828, 0.0096, 0.2563 and 0.0034 Debye, respectively. Also the alteration in the µD of ZnO-H_{2}BDC upon interaction with mentioned gas compounds increases in the order of CO_{2} > H_{2}. Our results reveal that more dipole moment (4.5891 for CO_{2}-ZnO-H_{2}BDC and 3.6621 for H_{2}-ZnO-H_{2}BDC) corresponds to more adsorption energy which is in agreement with the results of charge analysis.
To more understand the electronic properties of the organic ligand of C-MOF-5 and T-MOF-5 during adsorption, the density of states (DOS) as the electron density of HOMO-LUMO was calculated for H_{2}BDC and ZnO-H_{2}BDC and also their complex was formed with H_{2} and CO_{2} molecule (see Table 1). Quantum mechanically, the interaction between two reactants takes place because of the interaction of frontier molecular orbitals [37]. HOMO and LOMU have the aptitude to contribute electrons and detract electrons, respectively. However, if a molecule has high HOMO energy, then it will be more unstable and contrariwise because of more reactivity [38,39]. The HOMO and LUMO energy orbitals for H_{2}BDC and ZnO-H_{2}BDC are found to be (-7.38 eV, -2.12 eV) and (-6.18 eV, -3.10 eV), respectively. After ZnO doping of H_{2}BDC (Zn-H_{2}BDC) and adsorption of H_{2} and CO_{2} molecule on the surface of H_{2}BDC and ZnO-H_{2}BDC, some changes occur in electronic properties.
The HOMO-LUMO energy gap (E_{g}) is one of the key parameters to recognize the stability as well as the conductivity of resulted adsorptions. Higher E_{g} results in more stability and less conductivity of resulted complex. The band gaps of the pristine H_{2}BDC and ZnO-H_{2}BDC as well as complex forms of CO_{2}-H_{2}BDC-B, CO_{2}-H_{2}BDC-D, H_{2}-H_{2}BDC-A, H_{2}-H_{2}BDC-D, H_{2}-ZnO-H_{2}BDC CO_{2}-ZnO-H_{2}BDC are calculated and are listed in Table 1. Adsorption behaviors of H_{2} and CO_{2} on H_{2}BDC and ZnO decorated H_{2}BDC (ZnO-H_{2}BDC) are mixed. For pristine H_{2}BDC, the band gap is almost the same on H_{2} and CO_{2} adsorption but for ZnO-H_{2}BDC-H_{2} and ZnO-H_{2}BDC-CO_{2,} the band gap increases from 3.08 eV (for ZnO-H_{2}BDC) to 3.88 eV and 3.82 eV, respectively. The calculations revealed the highest E_{g} value for interacted ZnO-H_{2}BDC with H_{2} and CO_{2}, which again correlates well with the high adsorption energies of H_{2} and CO_{2} on ZnO-H_{2}BDC compared to H_{2}BDC.
The change in E_{g} leads to changes in conductivity. The relationship between conductivity and E_{g} can be given by [40]:
where is the electric conductivity, k is the Boltzmann constant and T is the temperature. According to this equation, a small decrease in E_{g} leads to significantly higher electrical conductivities. According to results, pristine H_{2}BDC is not a good adsorbent for H_{2} and CO_{2} (particularly for H_{2}) molecule but adsorption of ZnO on H_{2}BDC significantly enhances its ability towards H_{2} and CO_{2 }adsorption (particularly for CO_{2}).
Next, we analyzed the densities of states (DOSs) as well as the electron density of HOMO-LUMO to realize how the adsorption of ZnO affects the electronic structure of H_{2}BDC. Fig. 6 plots the DOSs as well as the electron density of HOMO-LUMO for H_{2}BDC and ZnO-H_{2}BDC near the Fermi level (E_{F}). According to Table 1 and Fig. 5, for pristine H_{2}BDC, the Fermi level (E_{FL}) is slightly changed from -4.75 eV to -4.79 eV, -4.76 eV, -4.79 eV and -4.75 eV for CO_{2}-H_{2}BDC-B, CO_{2}-H_{2}BDC-D, H_{2}-H_{2}BDC-A, and H_{2}-H_{2}BDC-D, respectively. For ZnO-H_{2}BDC, the E_{FL} is changed a lot from -4.64 eV to -3.92 eV and -4.08 eV for CO_{2}-ZnO-H_{2}BDC and H_{2}-ZnO-H_{2}BDC, respectively. After comparing the DOS of H_{2}BDC and ZnO-H_{2}BDC with that of their interacted forms, we found that the ZnO doping of H_{2}BDC (ZnO-H_{2}BDC) causes a significant shift of the occupied orbitals of H_{2}BDC to high energy levels and observed some change evidence of hybridization in the case of ZnO-H_{2}BDC systems.
Also according to Fig. 6, The HOMO and LUMO in H_{2}BDC are uniformly dispersed over the entire skeleton of the pristine organic ligand. The effect of H_{2} and CO_{2} adsorption on H_{2}BDC is almost negligible. The decoration of ZnO on H_{2}BDC brings quite significant changes regarding both energies and densities. Adsorption of ZnO on the surface of H_{2}BDC increases the energy of HOMO but decreases the energy of LUMO which ultimately results in a decrease in the band gap (from 5.26 to 3.08).
The effect of ZnO doping of H_{2}BDC (ZnO-H_{2}BDC) and H_{2} and CO_{2} adsorption on H_{2}BDC and ZnO-H_{2}BDC, and on the electronic structure of the H_{2}BDC, ZnO-H_{2}BDC, CO_{2}-H_{2}BDC-B, H_{2}-H_{2}BDC-A, CO_{2}-H_{2}BDC-D, H_{2}-H_{2}BDC-D, CO_{2}-ZnO-H_{2}BDC, and H_{2}-ZnO-H_{2}BDC can be seen in MEP map, shown in Fig. 7. According to Fig. 7(a and b), the pristine H_{2}BDC has symmetrical potential (due to symmetrical structure) and ZnO atom has positive potential. Also, as shown in Fig. 7(c,d,e and f) the adsorption of H_{2} and CO_{2} molecules on H_{2}BDC does not affect on the intensification of positive or negative charge of H_{2}BDC (particularly for adsorption on-site D Fig. 7(e and f)). For ZnO-H_{2}BDC, the Zn and O atoms have positive and negative potential (respectively) whereas the atoms in the H_{2}BDC have almost neutral potential. The negative and positive potential on H_{2}BDC was changed in ZnO decorated H_{2}BDC (ZnO-H_{2}BDC). Upon adsorption of H_{2} and CO_{2} (particularly for CO_{2}) molecule on ZnO-H_{2}BDC, the negative potential is regenerated on the H_{2}BDC. The MEP analysis also shows that upon H_{2} and CO_{2} adsorption, the position of positive potential also shifts to close around of Zn.
We have also calculated the global indices of reactivity for CO_{2}, H_{2}, ZnO, H_{2}BDC, ZnO-H_{2}BDC, CO_{2}-H_{2}BDC-B, CO_{2}-H_{2}BDC-D, CO_{2}-ZnO-H_{2}BDC, H_{2}-H_{2}BDC-A, H_{2}-H_{2}BDC-D and H_{2}-ZnO-H_{2}BDC systems in order to evaluate how the adsorption of ZnO affects the chemical properties of H_{2}BDC for H_{2} and CO_{2} adsorption (Table 2). The energies of LUMO correspond to the electron affinity (define as A) whereas the energies of HOMO can approximately be taken equivalent to ionization potential (define as I) [33]. The ionization potential (I) for pristine H_{2}BDC is very high and does not decrease upon adsorption of H_{2} and CO_{2}. The decoration of the H_{2}BDC with ZnO causes a reduction in ionization potential. Also, the ionization potential decreases when H_{2} and CO_{2} molecules are adsorbed on ZnO decorated H_{2}BDC. The band gap represents the softness (S) or hardness (η) of a compound. A compound with large band gap is expected to be hard, and conversely. After decoration of the H_{2}BDC with ZnO, the band gap decreases from 2.63 to 1.54 and it is expected that these complexes (ZnO-H_{2}BDC) are very soft. According to Table 2, after adsorption of H_{2} and CO_{2} on H_{2}BDC the softness values approximately remain unchanged, but after adsorption of H_{2} and CO_{2} on ZnO-H_{2}BDC, the softness values declined. The increase in chemical reactivity is electrophilic in nature because the electrophilicity of ZnO-H_{2}BDC (6.990 eV) is higher than the H_{2}BDC (4.289 eV).
CONCLUSIONS
We have shown theoretically for the first time that how H_{2} and CO_{2} adsorption on cubic and tetragonal structure of MOF-5 are different. Accordingly, we optimized the geometries for pristine H_{2}BDC as well as ZnO doped H_{2}BDC and their interaction with H_{2} and CO_{2}. Firstly we investigated the possible positions of ZnO on H_{2}BDC. Among the three sites around H_{2}BDC (site A, B, and D) for ZnO adsorbed H_{2}BDC, the site D is more stable than site A and B. According to obtained results, at least in the organic ligand of the MOF-5, the highest and lowest adsorption of CO_{2} and H_{2} is attributed to the T-MOF-5 and C-MOF-5 respectively. Also among the three sites around H_{2}BDC (site A, B, and D) by a change in structure from cubic (H_{2}BDC) to tetragonal (ZnO-H_{2}BDC), the main site for H_{2} and CO_{2} adsorption was changed from A and B to D site. Our calculations determine that ZnO-H_{2}BDC system (as a representative section of T-MOF-5) has much higher adsorption energy, and higher net charge transfer value than pristine H_{2}BDC (as a representative section of C-MOF-5).
This is a qualitative, exploratory study in which the authors are laying the groundwork for future researches on this subject. It is exploratory because no other researches have been conducted on this specific subject.
ACKNOWLEDGEMENT
The authors acknowledge Iran Nanotechnology Initiative Council for financial support.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.